Systems for detecting potentially hazardous airborne particulate matter in near-real-time can be used in military and civilian applications for nuclear, biological and chemical aerosols. In the nuclear industry, radioactive particulate continuous air monitors protect personnel in laboratories and industrial facilities. The U.S. military has field-deployable chemical and biological (CB) agent-detection systems to protect personnel in the event of a CB attack. Anthrax attacks experienced by the U.S. Postal Service in 2001 and the sarin nerve-agent attack in the Tokyo subway system in 1995 indicate a need for CB detectors in sensitive civilian locations. Although chemical agent and radioactive particulate detectors have matured through several design generations, practical biological point detection systems are relatively new, and significant advancements are needed before biological agent detectors perform on par with chemical agent and radioactive particulate monitors.
A major obstacle confronting biological detectors is the relatively low concentration of biological agent particles that can cause serious harm. In terms of the minimum detectable level in the sampled environment and reliability of the detector output signal, detector response can be enhanced by concentrating the sampled aerosol particles prior to detection. Concentration factors of 100 to 1000 are currently employed in detection systems. Sensitivity in future biological detection systems is likely to improve, which could potentially reduce the desired levels. Nonetheless, even future detection systems will benefit from aerosol concentration prior to detection, in that the greater number of organisms detected, the higher the probability of a statistically supportable alarm. For current and future applications, there is a critical need for small, portable, biological agent detection systems that are suitable for field applications and can efficiently concentrate airborne particles.
Virtual impaction is widely used for concentrating aerosol particles. The most common configurations in present virtual impactors are axi-symmetric, in which opposed acceleration and receiver nozzles are truncated-conical or round, and planar-symmetric in which the nozzles have opposed, inclined rectangular surfaces spaced apart to form slots with rectangular exits. The concept of virtual impaction can be understood from FIG. 1, which schematically illustrates an aerosol flow through an acceleration nozzle 1 and a receiver nozzle 2 of a virtual impactor. The aerosol (including particles suspended in a gaseous medium or gas phase) is drawn into accelerator nozzle 1 by a partial vacuum (negative pressure differential), and is accelerated by virtue of inclined surfaces 3 of nozzle 1 as it approaches an aperture 4. The aerosol flows longitudinally (vertically in the figure) through aperture 4 and into a fractionation zone in the gap between nozzles 1 and 2. As the aerosol flow enters the fractionation zone, negative or vacuum pressure is selectively applied to draw a major portion or fraction of the aerosol (in terms of volume per unit time) transversely away from the fractionation zone. The major flow, illustrated by streamlines 5, approaches nozzle 2 but undergoes a hairpin turn, doubles back toward nozzle 1, then flows into a diverging transverse exit passage. The gaseous medium and the smaller entrained particles tend to follow the path indicated by streamlines 5. In contrast, the larger particles tend to continue moving longitudinally into receiver nozzle 2, because they have momentum sufficient to overcome the tendency to flow with the gaseous medium.
Negative pressure also is applied through nozzle 2 to draw a minor portion or fraction of the aerosol flow longitudinally into the receiver nozzle. The minor flow, indicated by streamlines 6, passes through an aperture 7 into nozzle 2. Inclined nozzle surfaces 8 diverge to decelerate the flow. The gaseous medium and all particles of the minor portion tend to follow the longitudinal path indicated by streamlines 8.
Typically, the major flow constitutes about ninety percent of the original flow in terms of volume per unit time, while the minor flow constitutes about ten percent of the original flow. With the exception of losses due to deposition onto the walls near the fractionation zone, virtually all of the larger particles are transferred from the major flow to the minor flow, to provide a highly concentrated minor flow including about ten percent of the gaseous medium, ten percent of the smaller particles, and nearly all of the larger particles.
The larger particles and smaller particles are distinguished from one another based on a size threshold known as the cutpoint, i.e. the size at which particle momentum causes fifty percent of the particles to leave the major flow and merge into the minor flow. As particle sizes increase above the cutpoint, the percentage of the particles transferred from the major flow to the minor flow increases rapidly. Consequently, in polydisperse aerosols, virtually all of the larger particles are transferred to the minor flow, although very large particles may inadvertently be deposited on internal surfaces in the fractionation zone and thereby not transferred to the minor flow stream.
Aerosol (particle and gas phase) flow in a virtual impactor is governed primarily by two dimensionless parameters, the Stokes number (Stk) and the Reynolds number (Re). The Stokes number is given by:
                    Stk        =                                            C              c                        ⁢                          ρ              p                        ⁢                          D              p              2                        ⁢                          U              0                                            18            ⁢            μ            ⁢                                                  ⁢                          L              c                                                          (        1        )            where Dp is the particle diameter in centimeters, ρp is the particle density in kg/m3, Cc is the slip correction factor, U0 is the mean velocity at the expiration nozzle exit in m/second, Lc is the acceleration nozzle aperture dimension (radius of a circular nozzle aperture and half-width of a slot nozzle) in m, and μ is the dynamic viscosity of the gas in kg/m s.
The Reynolds number is given by:
                    Re        =                                            ρ              f                        ⁢                          L              c                        ⁢                          U              o                                μ                                    (        2        )            where ρf is the gas density in kg/m3, and the other values are as indicated above, except that the characteristic dimension Lc is the nozzle diameter for a circular nozzle and the nozzle width for a rectangular slot nozzle.
The Stokes number is the dominant parameter governing particle behavior in virtual impactors. The cutpoint Stokes number (Stk50) corresponding to the cutpoint particle size is weakly a function of the Reynolds number owing to minor differences in the flow field as affected by Reynolds number.
The pressure drop (ΔP) for moving air through the virtual impactor can be represented as a function of the acceleration nozzle throat velocity:
                              Δ          ⁢                                          ⁢          P                =                  K          ⁢                                                    ρ                f                            ⁢                              U                o                2                                      2                                              (        3        )            where: K is a pressure coefficient, essentially constant for a limited range of flow rates.
The ideal power (Pwr) required to operate a virtual impactor, i.e. the minimum power required to move air through the virtual impactor ignoring blower/pump inefficiencies and pressure losses in the associated flow handling system, is given by:Pwr=QmaΔPma+QmiΔPmi   (4)where: Qma is the major flow rate (of the fine particle flow in cm/sec); ΔPma is the difference between pressure at the entrance plane of the acceleration nozzle and pressure at the exhaust plane of the major flow in pascals (Pa); Qmi is the minor (coarse particle) flow rate in cm/sec; and ΔPmi is the difference between pressure at the entrance plane of the acceleration nozzle and pressure at the exhaust plane of the minor flow in Pa. Typically, the major flow components on the right side of Equation 4 are much larger than the minor flow components, because Qma is much larger than Qmi (e.g. by a factor of nine). Also, the pressure drop for the minor flow is negligible compared to the pressure drop for the major flow because of pressure recovery in the entry region of the receiver nozzle.
For bioaerosol concentration, the virtual impactor should have a cutpoint below the particle size range of interest. A bacterial agent like anthrax may consist of single-spores having aerodynamic diameters of about 0.9 μm. To achieve a cutpoint low enough to concentrate particles of this size with an acceptable level of power consumption, the virtual impactor must have the proper nozzle dimension (width or diameter) and mean nozzle velocity. For a given cutpoint the choices are (i) a larger nozzle dimension and higher mean nozzle velocity, and (ii) a smaller nozzle dimension and a lower mean nozzle velocity.
With a fixed cutpoint and flow rate, the ideal power to operate a virtual impactor is a function of nozzle width, increasing approximately with the square of the nozzle diameter or width (for a constant minor loss coefficient K). FIG. 2 is a plot of ideal power for operating a virtual impactor with a cutpoint aerodynamic diameter of 0.8 microns at a flowrate at 500 L/min (17.7 CFM), with a pressure coefficient K of 1.5. An impactor with a smaller nozzle dimension requires less power for a given cutpoint and flowrate. For example, a slit width of 0.254 mm (0.010 inches) requires an ideal power of 40 watts, while an impactor with a slit width of 0.762 mm (0.030 inches) requires 400 watts.
Present bioaerosol detection systems typically require flow rates in the range of 100 to 1000 L/min to reliably detect concentrations of biological agents expected in a release. For small dimension round-nozzle virtual impactors, these flow rates require an array of many nozzles. For slot nozzles, the total slot length must be sufficient to supply the required total flow, either as one continuous slot, or as an array of slots of intermediate length. Both approaches involve manufacturing difficulties, especially as the nozzle critical dimension approaches the level of tolerance control.
An array of many round nozzles increases the risk of producing defective nozzles, in that each nozzle requires small dimension chamfers and fillets. For slot nozzles, nozzle edge linearity and parallelism become more difficult to achieve as the nozzle dimension becomes smaller. Both designs require precise alignment mechanisms to align the centers of the receiver nozzle and acceleration nozzle. Also, both require considerable depth for the acceleration and receiver nozzles to gradually accelerate the aerosol particles approaching the fractionation zone and decelerate the large particles after fractionation. Thus, manufacturing processes such as photo-etching are of limited value.
As compared to round nozzles, slot nozzles are more resistant to fouling from debris. Round nozzles are more easily bridged by airborne fibers. Once a fiber bridges the nozzle, additional particles attach to the fiber, eventually fouling the nozzle and preventing proper function of the virtual impactor. Although slot nozzles can also become bridged by fibers, their long dimension allows them to avoid fouling to a greater degree. On the other hand, round nozzles are not subject to the inaccuracies introduced by disturbances at the opposite ends of the rectangular slots, known as end effects.
Neither the round nozzle design nor the rectangular-slot nozzle design is particularly well suited for a portable, compact aerosol particle concentration device with a minimal power requirement. In the case of round nozzles, this is due to the requirement for an array of nozzles to meet flow rate requirements. In the case of rectangular-slot nozzles, it is due either to the array requirement, or an inordinate length necessary to achieve a desired flow rate. Further, the nozzle interior in both designs leads to undesirable large-particle trajectory effects as the aerosol moves through the acceleration nozzle. More particularly and with reference to FIG. 1, large particles relatively close to one side of the nozzle can tend to travel transversely toward the opposite side of the aperture rather than flowing longitudinally through the aperture with the gaseous medium, due to particle momentum.
Therefore, it is an object of the present invention to provide an aerosol particle concentrating device that operates effectively at both micrometer and sub micrometer cutpoints, yet is compact and has low power requirements.
Another object is to provide a virtual impactor having an acceleration nozzle with a high ratio of slot length to slot width, which is not subject to end effects.
A further object is to provide an improved process for separating an aerosol into fractions with different particulate concentrations.
Yet another object is to provide a virtual impactor that promotes a more unidirectional flow of particles through the aperture of its acceleration nozzle.